Cellulose wadding, also designated as tissue paper, is a paper with a low gsm count, of which it is desirable, according to the intended uses, to have the qualities of softness, flexibility, strength and absorption, alone or in combination. Depending on the technique for manufacturing it, it may be of the creped type. For example, creping may be obtained at the time of drying of the sheet. The latter is applied, still moist, to a cylinder heated to a temperature sufficient for extracting the moisture from it. The sheet is detached by means of a doctor blade placed between the latter and the surface of the cylinder. This operation, suitably conducted, gives rise to the formation of corrugations arranged perpendicularly to the direction of travel of the sheet. The creping of the sheet is characterized by the number of crests counted in the running direction, reduced to a unit of length. These corrugations give the sheet some elasticity of which it would otherwise be virtually entirely devoid. The creped cellulose wadding thus possesses some elongation capacity.
This property makes it possible to emboss the sheet. The embossing operation involves deforming the sheet in the direction perpendicular to its plane, so as to increase its thickness. The sheet is embossed by driving it, for example, between a rigid-blanket cylinder, provided with spikes projecting on the surface, and a rubber-blanket cylinder. Other combinations are possible and known to a person skilled in the art. All such cylinders which perform this function will be designated hereafter by embossing group. Depending on the embossing intensity, a sheet of greater or lesser thickness is obtained, this thickness being measured between the two planes delimiting the sheet. Embossing also affects other physical parameters of the sheet, such as the tearing strength, which decreases by virtue of the break of connections between the fibers. Its elongation capacity likewise decreases by virtue of the plastic deformation undergone.
It is well known to combine two or more plies of cellulose wadding in order to form thicker, more absorbent or stronger sheets. They may be embossed beforehand in order further to improve the thickness and the absorption capacity or be given more bulk. To ensure the connection of the plies to one another, an adhesive film may be interposed between the surface portions which come into contact with one another when one ply is superposed on the other.
A mechanical connection may also be made by knurling. This operation involves rolling a knurling wheel, provided with a suitable relief, on the surface of a sheet consisting of the plies to be combined and in bearing contact on a rigid surface. The latter may be smooth or have a relief complementary to that of the knurling wheel. The knurling wheel is pressed down with sufficient force to generate a connection between the plies in the compressed zones. The knurling wheels and the rigid bearing surface, generally a steel cylinder, taken as a whole, will be designated hereafter by knurling group.
The applicant has produced products with two plies of creped cellulose wadding, of which one is embossed and the other not, It was found that, in some cases, transverse folds were formed, giving the sheet an unsatisfactory appearance.
The manufacturing method may be broken down into a plurality of steps, each of which is known per se. A first strip of creped cellulose wadding is unwound from a unwinder. It is guided through an embossing group, the pattern of which is composed, for example, of pin-shaped protuberances. A second strip is unwound from a reel. This may be a second reel independent of the first, but there may also be two strips wound on the same reel. In this case, a means is provided for separating the two strips.
The second strip is directed toward the first, downstream of the embossing group. They are superposed by being placed in such a way that the tops of the protuberances of the first are located within the assembly. Subsequently, or at the same time, a connecting means is applied, making the two strips integral with one another.
The connecting means may consist of a knurling group, when the plies are connected according to this technique, or else of a mating cylinder, when the plies are connected by the top of the protuberances of the first ply, generally by means of an adhesive film.
It was observed that the undesirable formation of the transverse folds or of blisters on one of the plies was linked to the presence of relatively wide zones where the plies are free of one another. This is particularly the case when the plies are connected by knurling along longitudinal lines arranged near the edges of the sheet after cutting to the format of use. This phenomenon is also observed when the plies are partially bonded adhesively. Adhesive bonding is referred to as partial when, for example, the number of the actual connection points is lower than the number of contact zones between the plies. This result is obtained, in particular, when the first ply is embossed with protuberances having different heights and when an adhesive film is applied only to the highest tops. More generally, this phenomenon is observed when the connected zones are spaced from one another.
One means for eliminating this disadvantage is to reduce the unconnected free spaces by increasing the density of the connected zones, that is to say their number per unit area. However, the rigidity of the sheet is increased correlatively. This then runs counter to the search for the flexibility which is appreciated by consumers.